The present invention relates to an electrophotographic photoreceptor which contains an azo compound containing a novel coupler component.
Hitherto, as photoconductive substances for electrophotogrpahic photoreceptors, there have been known and widely studied and used inorganic photoconductive substances such as selenium, cadmium sulfide, zinc oxide and silicon. These inorganic substances have many merits, but on the other hand have various defects. For example, selenium has the defects such as severe production conditions and tendencies to crystallize by heat or mechanical shock. Cadmium sulfide and zinc oxide are insufficient in moisture resistance and endurance. Silicon is said to be low in chargeability and have difficulties in its production. Selenium and cadmium sulfide have the further problem of toxicity.
On the other hand, organic photoconductive substances have advantages such as good film-formability, superior flexibility, light weight, high transparency and easy design of photoreceptors applicable to a wide wavelength region by using proper sensitizing methods and are increasingly put to practical use.
Fundamental properties required for electrophotographic photoreceptors include (1) high chargeability when exposed to corona discharge in the dark, (2) less leakage, in the dark, of the charge obtained (dark decay), (3) rapid release of the charge when exposed to light (light decay) and (4) little residual charge after exposed to light.
Although many researches have been made on photoconductive polymers such as polyvinylcarbazole to be used as organic photoconductive substances, these are not satisfactory in film-forming properties, flexibility and adhesiveness and do not fully meet the above mentioned fundamental requirements.
On the other hand, photoreceptors excellent in mechanical properties such as film-forming properties and flexibility can be obtained with organic low molecular photoconductive compounds by selecting binders used for preparing photoreceptors, but it has been difficult to find compounds suitable to maintain the characteristic of high sensitivity.
In order to solve these problems, organic photoreceptors having higher sensitivity have been developed by allotting the carrier generation function and the carrier transport function on separate materials. The characteristic of these photoreceptors which are called function-separated type photoreceptors is that materials suitable for the respective functions can be selected from a wide variety of materials and photoreceptors having optional performances can be easily produced. Thus, intensive research has been made thereon.
As stated above, development of novel materials or combination of them and various improvements have been made for meeting the requirements such as fundamental properties and endurance demanded for electrophotographic photoreceptors. However, the improvements have not yet reached sufficient levels.
One serious problem is that when the photoreceptors are actually used in a copier, they undergo a strong oxidizing action of ozone generated from a charger. Photoreceptors comprising organic materials readily undergo this oxidizing action to cause decrease in chargeability or sensitivity. For solving this problem it has been proposed to add various antioxidants to the photoreceptors. However, such method is not necessarily effective and rather causes problems such as deterioration in sensitivity and increase in residual potential. As a result of extensive research on deterioration of photoreceptors conducted by the inventors, it has been found that the oxidation reaction in the area at which a carrier generation material and a carrier transport material contact exerts a serious adverse effect upon the electrophotographic characteristics of the whole photoreceptor and it has been elucidated that the oxidizing action can be effectively inhibited by using azo compounds containing a specific coupler component having an antioxidizing action.